1. Field of the Invention
This invention relates to machine tools for shaping or finishing objects of complex shape to conform to a predetermined computer model. More particularly, it relates to a grinding machine having six axes under simultaneous computer control that permit simplified computer control programs and automatically produces objects of complex shapes rapidly and accurately. The described embodiment of the invention is particularly adapted to produce airfoil shaped blades and buckets, for example, of the kind used in turbines, and other objects having complex curved surfaces.
2. Description of Related Art
Objects of complex shape, such as turbine blades and buckets, are generally manufactured in several stages. In the first stage, a rough form is created by casting, forging, or milling by pantographic or numerically-controlled equipment. In the pantographic milling process, a cutter blade is controlled to follow the path of a tracing tool as it makes successive peripheral paths around a prototype. In a numerically-controlled milling process, a ball-end milling cutter is caused to follow a predetermined path to generate a near-final shape.
In milling processes, manufacturing economy dictates that the distance between adjacent passes around the periphery of the workpiece is relatively large. The workpiece surface, as it is generated by milling, is not smooth but consists of numerous nearly-parallel shallow grooves that correspond to the path of the cutting tool. The as-milled workpiece is not smooth enough or close enough to the required dimensions to have practical application. Cast or forged parts have inherently rough surfaces and coarse manufacturing tolerances, unless the parts are made by expensive precision casting processes, such as investment casting.
The cast or milled workpieces must be further refined in a second stage to smooth the surface and to bring it within permissible dimensional tolerances. There are several variations of these processes, but all are expensive and result in a significant rejection rate because of dimensions that are outside of acceptable tolerances. Automatic machining using computer controlled equipment has not been commercially practicable because of the complexity of the necessary programs to operate the finishing equipment using the current milling techniques.
U. S. Pat. No. 4,369,563 describes a computer-controlled machine system in which a number of separate numerically controlled machines perform successive machining operations on a workpiece that is moved automatically from one station to another.
U.S. Pat. No. 4,031,809 employs an end milling machine in which a plane end of a milling element is maintained approximately parallel with the plane of the surface being shaped. However when machining concave surfaces, such as those of turbine blades, the axis of the milling element is purposely skewed with respect to the local normal to avoid undercutting the surface. The milling device disclosed does not provide a smooth surface finish within the necessary tolerances for many applications.